Considering Copperhorn Scout, this card is absolutely awful. Plus, I'm not sure why they have to have a Plains.

The plains thing was part of a cycle with (((Pondscum Newt))), Murklurk Stag Beetle, Altitude-Apt Alpaca, and Forest-Runner Tapir.

If the idea is to punish people with Plains, that's absolutely fine. What I meant was that the additional restriction seemed out of place for an effect that was already overcosted.

"Tapping or untapping" isn't supposed to be white (although I'll be honest I'm not sure about untapping's distribution in the color pie as of right now). If I can suggest something, try putting a 1/1 token. (I'll admit finding small white effects to happen AFTER combat are very hard.)

About the cycle in general, it's very lackluster. You can do many things to make them more relevant and still keep the fact that they are conditional and common.

Maybe all of them can have some sort of evasion, or at least an ability that doesn't let them crash and die when blocked by anything. However, this makes them very high-variance. If you give the blue one flying, for example, it's a Sea Eagle against many decks, but the sickest Scroll Thief against blue.

So maybe the evasion can be dependent on the basic land of an opponent, and the ability can work without condition. Like, this 1/1 has trouble getting through but when it can it's worth it. And if the condition is met, then it's strong overall. That makes the card the same against opponents who have the correct basic land, but more exciting in other cases. And because of this it would be easier to balance.

In either case, you still fall in the "landwalk is too swingy" problem. Somberwald Dryad is a fine girl, and it's a Phantom Warrior against some opponents. Probably manageable. Oh, wait, you have two, I guess I have no chance. All I wanted was to cast my green cards, shame on me.

There's nothing faulty with your cycle in particular, and you may be okay with some situations making these cards very good, but both try not to make them too powerful even there (Murklurk Stag Beetle is impossible to race if it gets evasion of any kind), and don't make these seem like overcosted Memnites that do nothing against many players.

I have a weird color-based ability cycle in my Mirrormarked creatures cycles. White's two for reference are Mirrormarked Ocelot and Mirrormarked Frazzlemane.

I think destroying enchantment is much more reasonably costed. However, be careful with clumping a card with restrictions. This needs your opponent to control a Plains, an enchantment, and not have anything with at least 2 toughness to block. And even if they have a 1/1 it's probably better for them to trade. It's like a quest where you don't control what happens.

About your other cards, they are much better in terms on being okay bodies when you don't have the benefit and good when you do. But I would strongly recommend you not to have three creatures with similar-but-different triggers. People will make mistakes all the time about what things happen when. I'd say only 1 self-hate card per color is enough. If you want that to be a big theme, find other ways to show it. I'd keep one "blocks or becomes blocked by [color] creature", maybe have a cycle of creatures that only has landwalk of their color, and a spell that targets an opponent's creature, and then does something additional if it's of the same color.

This cycle is no more.

This is not an effect for a common. See this gatherer search.